1. Field of the Invention
The invention relates to a process and a device for segmentation into sub-bands and for reconstruction of a digital signal.
2. The Prior Art
Presently, there are several techniques for the splitting or for the transformation of digital signals, from the time domain into the frequency domain, and for the corresponding associated inverse transformation. Among those best known may be cited the discrete cosine transform, or DCT, the fast Fourier transform, FFT, or the Karhunen-Loeve transformation. Furthermore, certain techniques are more specific to splitting into sub-bands and to the corresponding reconstruction, these techniques being more especially based on the use of conjugate quadrature filters or CQF, quadrature mirror filters, QMF and pseudo quadrature mirror filters or PQMF.
Generally, these techniques for transformation and reconstruction are used for example for the purpose of performing a throughput reduction coding of digital signals, made possible by concentrating the useful information over a reduced number of mutually uncorrelated coefficients, which are then transmitted alone.
These techniques can be and are used for the transmission of digital audio or video signals for example.
However, the above techniques other than the processes for segmentation into sub-bands, applied to the transmission of digital video signals, have the disadvantage of applying one set of coefficients to a block of pixels, taking no account of the adjacent blocks. The effect of such a lapse, during the procedure for reconstructing the original signal after transmission, is to generate block effect phenomena which are all the more visible the lower the number of useful coefficients arising from the transformation.
By way of illustration may be cited, as represented in FIG. 1a, the example of a DCT transform applied to a digital signal, linearly dependent on the rank of each block, for which only the continuous coefficient (or component) is preserved in the frequency domain, following the transform carried out. The signal generated after reconstruction then takes the form of a staircase function, the length of whose plateau or step is equal to the dimension of the DCT transform used. Such a function reveals, in particular, the effects of blocks inherent in the use, for the implementation of this type of transform, of weighting or filtering windows of type logical AND [1,0] over the length of each block, as represented in FIG. 1b.
Elimination of the aforesaid block effect has been made possible by implementing the processes for sub-band splitting/reconstruction using filtering windows with softer profile and of greater length than the length of the blocks, and such as represented in FIG. 1c, and thereby deriving more from the correlations between neighboring blocks.
As regards the sub-band filtering processes implemented using cascaded structures based on CQF or QMF filters, the obtaining of a near-perfect reconstruction and the involvement of a consequent number of sub-bands have led to the use of relatively long filters requiring prohibitive computation times.
In the case, on the contrary, of transformations implementing banks of PQMF filters generating sub-bands in parallel, although the necessary computational costs are not as large, the length of the filters is by contrast large, at least equal to eight times the number of segmentation sub-bands, whereupon, employing certain approximations however, almost perfect conditions for reconstruction of the original signal are sought. The use of PQMF filters with 64 coefficients is thus common for the splitting and then reconstructing of a digital signal as eight sub-bands, the filters used for this type of transformation most often consisting of an even number of coefficients, even multiples of the number of sub-bands SB.
Generally, as represented in FIG. 1d, a device for segmentation into sub-bands of conventional type comprises a plurality SB of parallel paths each receiving the samples E(n) of the digital input signal. Each of the paths corresponds to a sub-band and includes a digital sub-band filter, denoted 100-1, for the path or the sub-band of corresponding rank k. This digital filter is formed for example by a prototype filter of low-pass type, modulated by a periodic function. It will however be noted that the prototype filter of low-pass type with transfer function h(n) can be replaced by a high-pass filter with transfer function (-1).sup.n .multidot.h(n). It is followed by a sub-sampler circuit, denoted 100-2, this circuit performing a sub-sampling by the number SB of sub-bands or of paths so as to deliver a sub-band signal X.sub.k for each path of corresponding rank k.
It is of course understood that each path thus includes a digital filter followed by a sub-sampler circuit in order to deliver a sub-band signal of corresponding rank k.
Similarly, as likewise represented in FIG. 1d, a device for reconstructing a digital sub-band signal of conventional type, this signal corresponding to a plurality SB of sub-band signals X.sub.k delivered in parallel following an earlier segmentation into sub-bands of a digital input signal E(n), comprises a plurality SB of parallel processing paths, each parallel path, for a relevant sub-band signal X.sub.k, including a circuit for sub-sampling by the number SB of sub-band signals, this circuit being followed by a digital sub-band filtering circuit. This circuit is formed of a prototype filter of low-pass type modulated by a periodic function in order to deliver a sub-band component, denoted R.sub.k, of the reconstructed digital signal, obtained by summing the components.